Today we woke up to this! Global warming is too cruel!
In any case, it was bound to be a slower day, and I took the time to work a bit. However, I did take notes on one talk:
Branka Ladanyi (Colorado State, USA) "Liquid confined confined in silica nanopores". It's a simulation study on MCM-41. Pores of diameters 20 - 40 A. Calculated properties that can be compared with experiment: QENS (Quasi-Elastic Neutron Scattering) and OKE (Optical Kerr Effect). Pores in simulation prepared as in Gulmen & Thompson, Langmuir 25, 1103 (2009), with OH groups on walls at density of 2 - 2.5 per nm^2. Studied using 2-box Gibbs ensemble MC simulations. Using the filled pore density obtained in Gibbs ensemble, perform NVT simulations of single filled pores at that density. Simulation box is 60x60x40 A and diameters of 20, 30 and 40 A. Using SPC/E water. Results are more or less what you would expect:
* Density profiles show density in interior 90% of bulk. Since pores are rough, oscillations near wall is washed out. Slight density enhancement near wall is still visible. Consistent with A. Soper.
* Hydrogen-bond density (n_HB / A^3) is a bit below bulk water (about 0.11 vs 0.12). Slight peak in hydrogen bond density near wall due to water-silica H-bonds
* MSD along cylinder axis is linear with time (perpedicular to cylinder axis, trivially saturates). You can model analytically the diffusion in confinement in a cylinder. At long time scales, can fit diffusion constants. In bulk, D = 2.49 x 10^-9 m^2 / s, in confinement, D ~ 1.55 for R = 10, 1.80 for R = 15 and ~2.2 (?) for R = 20 A. [ask about finite size effects on diffusion].
* If you split pore into core, surface and outer layers, find no preferential orientation in core, but orientation correlates to surface normal near surface.
* If you look at diffusion only inside core, only inside surface, find different diffusion constants (faster diffusion in core).
* Non-exponential decay in orientational correlation function C_1(t) = <P_1(u(t) . u(0))>, slower for narrower pores. If you look only in the core region, relaxation is much faster and exponential, and equal for different diameter tubes.
* Laage & Thompson (JCP, 2012) find power-law in C_2(t) at long times for water in hydrophilic silica pores.
She then went on to map out how these observations are reflected in the experimentally-measured self-intermediate scattering function, F_s(Q, t).
With the Optical Kerr Effect, can measure polarizability anisotropy time-correlation: Psi(t) ~ < Pi^xz(0) Pi^xz(t)>, where Pi = Pi^Molecular + Pi^Induced, Pi^Molecular is just a sum of the polarizabity tensors alpha of individual molecules, and Pi^Induced is an infinite sum whose first term is something like alpha_i . T(r_ij) . alpha_j, where T(r) is the dipole-dipole interaction tensor. I guess what's going on here is that under an applied electric field E, the dipole moment of the system is M = Pi . E. God knows how the experiment measures <Pi^xz(0) Pi^xz(t)>.
In the afternoon, I had an interesting conversation with Werner Kuhs on how dewetting might play a role in the surface structure of methane hydrates: maybe something interesting will come of it.